QuestionHi, I am a proud owner of a 91' Cadillac Sedan Deville 4.9L which has 253,542 miles on it. My question today is I replaced the outside temp. sensor and the sunload temp. sensor and my car still tells me it's -40 outside. I've tryed clearing the codes which where F10 and F15 by pressing off and Lo to display F.0.0 but when I turn the ignition off and then back on to see if the codes are gone they reappear. I've even tryed unplugging the battery for a day or two then plugging it back up only to found that E52 which I cleared and F10 and F15 to be present. One automotive shop said that it might be my computer but they didn't say which one ECM or BCM. So I will leave this problem to you:) My next issue is the idle. Every time I start my car and it gets to normal operating temp. the idle starts going in and out like the engine wants to cut off which it usally does. I've replace the TPS sensor my self, just don't know if it's at the right rpm settings and I've also replace the ISC motor. My computer donesn't give me any codes besides the F10 and F15 . I've had major work done to the engine they consist of:) transmission overhaul, isc, tps, map, alternator, starter, battery about 2 weeks ago, engine and trans. mount, water pump, fuel pump, radiator, fuel filter, cataliytic converter, new distributor and all parts dealing with the distributor, and of course new spark plugs:)
AnswerHi klay, Congrats on being able to get that many miles out of your Deville. Some people are lucky to get even 125K.
As for your questions. The first one about the outside temperature reading -40. That happens when the sensor is unplugged or one of the wires to the sensor has gone open. The computor that controls it is the BCM and there is a possability it could be defective. You would need to get ahold of a shop manual and go through the testing to determine what is causeing the reading. If your EGR valve has a weak spring that could do it also. To rule it out just unplug the vacuum hose and plug it with a golf tee and see how the engine reacts.
The idle fluctuation is going to be alittle more difficult. There is a special idle learn procedure that has to be followed or the idle could do what its doing or the engine could idle too fast. It is also critical that the TPS and ISC motor also be adjusted correctly along with the base idle. Another thing that could be a problem is some of your fuel injectors could be shorting out. A good way to tell is if the idle when cold is smooth but as the engine reaches operateing temperature it will start to run rougher then you have at least 1 bad injector.
If you can't get ahold of a shop manual eithor from the parts store or your local library you might want to subscribe to ALL DATA.com and that way you will have access to all the info needed to do these adjustments.
Hopefully that will get you in the right direction otherwise taking your car to the shop would be a good idea. Bill
